1. Field of the Invention
The present invention relates to a sheet conveying apparatus to be included in an image forming apparatus such as a copier, a printer, a facsimile, and so forth, and an image scanning apparatus such as a scanner, and so forth.
2. Description of the Related Art
In recent years, with image forming apparatuses, there has been demand for high productivity (the number of sheets on which an image can be formed per unit time), and demand for reduction in size thereof, and also there has been increased demand for correcting skewing or misalignment of sheets with high precision, to improve image quality. Further, regarding thickness of sheets to be employed, it is necessary to convey thin paper to thick paper, small sizes such as a postcard to large size of 330 mm×488 mm or so regarding sizes, and further various types of paper such as coated paper, embossed paper, and so forth regarding paper type, in addition to plain paper.
First, in order to realize high productivity, it is effective to reduce the interval of sheets to be conveyed (the distance between the trailing edge of the preceding sheet and the leading edge of the next sheet) as short as possible (short paper interval). Along with this, it is necessary to correct the skew and misalignment of sheets occurring at the time of supplying sheets over a short period of time.
Consequently, as for a skew correction unit configured to correct the skew of a sheet, a method arranged to correct the skew of a sheet while conveying the sheet has been proposed instead of an existing method arranged to correct the skew of a sheet by contacting the tip of a sheet against the pressing portion (nip portion) of a stopped roller pair. This technology has been disclosed in Japanese Patent Laid-Open No. 1992-277151. This skew correction method is a so-called active registration method, which has an arrangement such as shown in FIG. 17, for example.
As shown in FIG. 17, two sensors 1101a and 1101b are disposed in the direction (sheet width direction) orthogonal to the sheet conveying direction (a sheet proceeds from the left to the right in the drawing), and detect the tip of a sheet S to be conveyed. Subsequently, the amount of skew of the tip of a sheet S is calculated based on a detection signal when the sheet S passes through the sensors 1101a and 1101b. Subsequently, according to a skew correction roller pair 1103a and 1103b which are disposed on the same shaft in the sheet width direction with a predetermined interval, and are independently driven and controlled by motors 1102a and 1102b respectively, the skew of the sheet S is corrected depending on the amount of the calculated skew. Thus, the skew can be corrected even in the event that the interval between sheets is short.
As shown in FIG. 18A, when assuming that the conveying velocity at the time of correction by the skew correction roller pair 1103a and 1103b are VL and VR, the thrust pitch between the skew correction roller pair is LRP, and the rotation velocity at the rotation center o of the sheet S is ω,
                    ω        =                                            V              R                        -                          V              L                                            L            RP                                              [        1        ]            holds.
As shown in FIG. 18B, when assuming that the rotation radius from the rotation center O of the sheet S to the center o′ between the skew correction roller pair 1103a and 1103b is RROT, according to “RROT·ω≡(VL+VR)/2”,
                              R          ROT                =                                                            V                L                            +                              V                R                                                    2              ⁢                                                                                    V                    R                                    -                                      V                    L                                                                                                ·                      L            RP                                              [        2        ]            holds.
According to that the force applied at the time of correction by the skew correction roller pair 1103a and 1103b are FL and FR, the conveying load (back tension) applied to the sheet S is FBT, and the relation of force balance is “FL+FR+FBT≡0”,FLx+FRx+FBTx=0FLy+FRy+FBTy=0hold.
According to that from the rotation center o of the sheet S to the skew correction roller pair 1103a and 1103b are RL and RR, from the rotation center o of the sheet S to a conveying load point is RBT(X, Y), and the relation of moment balance is “RL·FL+RR·FR+RBT·FBT≡0”,−(RROT−LRP/2)FLx−(RROT+LRP/2)FRx+(XFBTy+YFBTx)=0holds.
The conveying load FBT applied to the sheet S is ideally applied in the opposite direction of the rotation conveying direction of the sheet S, so according to “RBT⊥FBT”,XFBTx+YFBTy=0holds.
Accordingly,FLx=−1/2·[1−2/LRP·{(X2+Y2)/Y·RROT}·FBTx  Expression (1)FRx=−1/2·[1+2/LRP·{(X2+Y2)/Y·RROT}·FBTx  Expression (2)hold.
In general, it has been known that when assuming that the ideal maximum conveying force of the skew correction roller pair is F0, and the conveying load applied to the sheet S is FBT, the slip ratio of the rollers can be represented with a function of FBT/F0 experimentally. When assuming that ideal velocity is V0, actual velocity V, and a slip ratio function is F(FBT/F0), the conveying velocity at the time of skew correction isV=(1−F(FBT/F0))V0 
When assuming that control time is τ, and the amount of slip is d,d≈(V0−V)τ=F(FBT/F0)V0τholds.
It has been known that when the roller slip d is sufficiently small, the slip ratio function is experimentally capable of approximating a proportional expression, so when employing a constant k,d≈k(FBT/Fmax)V0τholds.
Skew correction accuracy becomes the difference |dLx−dRx| of the amount of slip between the skew correction roller pair 1103a and 1103b, |dLx−dRx|≈k(|FLxVLFRxVR|/Fmax)τ  Expression (3)holds.
Thus, the force FLx and FRx applied at the time of correction by the skew correction roller pair 1103a and 1103b is proportionate to the conveying load FBT as to the sheet. Accordingly, in general, a configuration has been employed wherein all of the upstream-side conveying rollers shown in FIG. 19 of the skew correction roller pair 1103a and 1103b are separated to turn a conveying guide into a straight path, thereby reducing the conveying load FBT as to the sheet.
However, turning the conveying guide of a registration portion into a straight path leads in a problem wherein the whole apparatus increases in size. Accordingly, in order to reduce the size of the apparatus, it is necessary to dispose a bent conveying guide upstream of the registration portion, but the bent conveying guide causes the conveying load FBT as to the sheet to increase drastically.
Additionally, in recent years, the grammage of sheets to be employed is diversified into various kinds from thin paper of 50 g/m2 or so to thick paper of 300 g/m2, and the size of sheets is also diversified into various kinds from a small size of a postcard or so to a great size of 330 mm×488 mm or so. In particular, when correcting the skew or misalignment of large-sized sheets employing thick paper having great inertial force, the conveying load FBT as to the sheet further increases. Thus, the force FLx and FRx applied at the time of correction by the skew correction roller pair 1103a and 1103b also increases pro rata. The force differences with the force FLx and FRx applied at the time of correction increase simultaneously (Expressions (1) and (2)), which leads in a problem wherein skew correction accuracy is deteriorated by slip of the skew correction roller pair (Expression (3)).
Further, as for the types of sheets to be conveyed as well, in addition to plain paper, various types of paper such as coated paper, embossed paper, and so forth have been requested. Therefore, with a sheet feeder configured to separate and feed the uppermost sheet of a loaded sheet bundle one at a time, so called “air sheet feeding” is frequently employed wherein air is sprayed on the loaded sheet bundle to compulsorily separate the sheet bundle one at a time. Thus, various types of loaded sheet bundle can be separated and fed one at a time, but there is a possibility that the sheets on which air was sprayed are greatly skewed. Therefore, in order to correct the great skewing of the sheets with a short conveying path length in a short period of time, it is necessary to increase the velocity difference as to the conveying velocity VL and VR at the time of correction by the skew correction roller pair 1103a and 1103b (equivalent to reduce the rotation radius RROT). In addition to the velocity difference as to the conveying velocity VL and VR, the force difference as to the force FLx and FRx applied at the time of correction by the skew correction roller pair 1103a and 1103b also increases simultaneously (Expressions (1) and (2)), which leads in a problem wherein skew correction accuracy is deteriorated by slip of the skew correction roller pair (Expression (3)).